1. Field
The described technology generally relates to wireless charging, and more specifically to devices, systems, and methods related to allocating power to receiver devices that may be located in wireless power systems.
2. Background
Typically, each battery powered device such as a wireless electronic device requires its own charger and power source, which is usually an alternating current (AC) power outlet. Such a wired configuration becomes unwieldy when many devices need charging.
Approaches are being developed that use over-the-air or wireless power transmission between a transmitter and a receiver coupled to the electronic device to be charged. Such approaches generally fall into two categories. One is based on the coupling of plane wave radiation (also called far-field radiation) between a transmit antenna and a receive antenna on the device to be charged. The receive antenna collects the radiated power and rectifies it for charging the battery. Antennas are generally of resonant length in order to improve the coupling efficiency. This approach suffers from the fact that the power coupling falls off quickly with distance between the antennas, so charging over reasonable distances (e.g., less than 1 to 2 meters) becomes difficult. Additionally, since the transmitting system radiates plane waves, unintentional radiation can interfere with other systems if not properly controlled through filtering.
Other approaches to wireless energy transmission techniques are based on inductive coupling between a transmit antenna embedded, for example, in a “charging” mat or surface and a receive antenna (plus a rectifying circuit) embedded in the electronic device to be charged. This approach has the disadvantage that the spacing between transmit and receive antennas must be very close (e.g., within thousandths of meters). Though this approach does have the capability to simultaneously charge multiple devices in the same area, this area is typically very small and requires the user to accurately locate the devices to a specific area.
For many wireless charging systems, the power transmitted from the source is fixed to a single level, thus the power level generally cannot be adjusted to accommodate devices with different maximum peak power levels. This limits the type of devices that can be charged. Another problem is that fixed radiated power levels cannot be adjusted as a function of the device's current battery level. This wastes power since as the battery charges it needs less and less power to complete the charge. Radiated power from the transmitter that is not absorbed by the device can increase Specific Absorption Rate (SAR) levels. A fixed transmitter power dictates that SAR requirements must be met for the worst case which occurs when the device being charged has poor coupling to the transmitter. Hence, a device with good coupling is limited to the power levels dictated by devices with poor coupling, which can lead to increased charge time for that device. When charging multiple devices, a fixed transmit power implies the same power level must be applied to all devices, no matter what charge level is optimum for each device. As mentioned earlier, this can result in wasted radiated power.
With wireless power transmission there is a need for apparatuses and methods for transmitting and relaying wireless power at varying power levels and multiplexed times to increase power transmission efficiency.